S12-1 ADM740, Section 12, June 2007 Copyright 2007 MSC.Software Corporation SECTION 12 USING...

S12-1ADM740, Section 12, June 2007Copyright 2007 MSC.Software Corporation

SECTION 12

USING FLEXIBLE BODIES


● In this section, you will learn how to create flexible bodies in your models, as well as how to swap a rigid body for a flexible body.

Using Flexible Bodies


● What’s in this section:● Flexible Body Overview● Limitations of Flexible Bodies● Getting Flexible Bodies● Modal Superposition● Visualization Attributes● About Joints and Motions● Joints Connection Limitations

Using Flexible Bodies


● Adams/Flex uses an assumed-modes method of modeling flexible bodies, called modal flexibility. Modal flexibility assigns a set of mode shapes to a flexible body. This modal method of modeling flexibility can be very useful in problems that are characterized by high elasticity and moderate deflections. That is, deflections less than 10% of a characteristic length of the body.

Flexible Body Overview


● By integrating flexible bodies into your model, you can:● Capture inertial and compliance properties during handling and

comfort simulations.● Predict loads with greater accuracy by allowing MD R2 Adams to

account for flexibility during simulations.● Study deformation (stress/strain can be calculated using

Adams/Durability).● Examine the linear system modes of a flexible model when you

use Adams/Flex with Adams/Linear.

● You should use flexible bodies wherever you expect component flexibility to affect the dynamic behavior of your model or when you require accurate information about the deformations of a component in your model.

Flexible Body Overview (Cont.)


● When you use flexible bodies, remember that flexible body deformations are a linear combination of deformation shapes. Consequently, take special precautions when modeling higher order deformations, such as those that occur when deformations are large, or when attempting to correctly model centrifugal stiffening of rotating systems. You can overcome these limitations by dividing a flexible body into multiple flexible bodies and assembling them in Adams/Car.

● Also, note that flexible bodies are not parametric. If you want to substitute a new flexible body in your system, you must create a new flexible body.

Limitations of Flexible Bodies


Getting Flexible Bodies

● Two ways to create flexible bodies● Importing modal neutral files (.mnf) - To create a new flexible

body, go to Build Part Flexible Body. Adams/Car imports the .mnf file and creates the flexible body.

● Creating .mnf files with Adams/Flex - With the additional module (it requires a separate license), a flexible body can be generated without access to an external FEA package. Specify the cross section, center line, and attachment points. This tool generates the flexible body, just like importing an .mnf file. In Adams/Car, Adams/Flex will also allow you to create a truck frame (two rails with multiple cross-members).

For more information on Adams/Flex, see the Adams/Flex online help.


● Adams/Flex treats flexible body deformations as small, linear deformations relative to a local reference frame undergoing large motion

● Represent deformation as a linear combination of mode shapes

● Simple example

● Craig-Bampton modes distinguish boundary nodes from interior nodes

Modal Superposition


● Fixed boundary normal modes

● Constraint modes

For more Adams/Flex theory, see the Adams/Flex online help (specifically, see the index entry Craig-Bampton modes - considerations when translating FE models).

Modal Superposition (Cont.)


● You can use the Plot Type option to display contour or vector plots.

Visualization Attributes


Picture

● Color contours● Indicate the deformation magnitude of the flexible component● Are continuous attributes, not discrete nodal values● Show relative deformation, not stress● Contours turned on in Adams/PostProcessor with the Contour

Plots tab

Visualization Attributes (Cont.)


● Deformation scale factor● Used to exaggerate deformation● Can scale up or down● Constraints can appear violated when the scale is >1. This is

merely visual: the analysis will, of course, maintain the constraints you’ve defined.

● When the scale is equal to 0, the shape will not be deformed but the colors will be displayed.



● Datum node● Deformation is a relative term and should be expressed with

respect to a node:● You select which node Adams/Flex considers as the undeformed

(datum) node and then all other nodes deform relative to it● Nodal deformations are colorized relative to the datum node● LBRF (local body reference frame, the default, also known as the

body coordinate system or BCS) is in the same location as the reference frame used in FEA



Node 1000

Node 1001

Node 1000

Node 1001

● Example of using different datum nodes● Here, node 1000 is the datum node.

● In this case, node 1001 is the datum node.

● The color red denotes maximum deformation relative to the datum node.



About Joints and Motions

● Using joints● After you bring a flexible body into Adams/Car, you can connect it

to your rigid model using the Adams/Car library of constraints.

● Joint locations● The joints directly connected to a flexible body must be at node

locations.● It is not required that the node be an attachment point in FEM.

However, it is good modeling practice to connect joints at attachment points.

● You can avoid nodal mismatch by:● Using consistent numbering● Paying attention to alignment issues


Joints Connection Limitations

● Joints are commonly used to attach flexible bodies● Fixed● Revolute● Spherical● Universal (or Hooke)

● You cannot directly connect some joints to flexible bodies● Joints that are being driven by motion generators● Joints permitting any translational motion (translational, planar, and

so on)● Joint primitives permitting any translational motion (inline, inplane,

and so on)Workaround: Attach the joint to an intermediate dummy part (for example, an interface part) that is fixed to the flexible body at a node.

S12-1 ADM740, Section 12, June 2007 Copyright 2007 MSC.Software Corporation SECTION 12 USING...

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